Method of producing nitrogen containing cases



Patented Aug. 1, 1933 METHOD OF PRODUCING NETRQGEN CONTAINING CASES John J. Egan, Brooklyn, N,Y., assignor to Electra Metallurgical Company, a Corporation oi West Virginia No Drawing. Application May it, 1929- Serial No. 363,113

10 Claims.

My invention relates to the production of hard,

wear-resistant, nitrogen-containing surface layers or cases on ferrous materials.

These nitrlded cases are usually produced on ferrous alloy articles that contain a critical amount of an alloy constituent which promotes the formation of the coating at comparatively low temperatures by heating the article in an atmosphere of ammonia. Aluminum is one of the known alloying or promoting agents. Temperatures of about 460 C. to about 580 C. are usually used but higher or lower temperatures may sometimes be used. At these temperatures, the production of the coating requires a treatment of about 12 to 90 hours. The time consumed in the process requires the use of a large amount of apparatus per unit of product and means for accelerating the action of the nitriding gases that are used for producing the cases, are desired.

It has been found that substances which give on nitrogen-containing decomposition products at or below the temperatures used for nitriding will accelerate the formation of the nitrogencontaining coatings. These accelerators may be used in conjunction with an inert material and a nitriding gas, such as ammonia. The solid cyanides and amides are suitable for this purpose since they start to decompose/at or below the temperatures used for nitriding.

For example, urea, alkali cyanides and alkaline earth cyanamides may be used over a wide range of temperatures that are higher than the temperatures at which they start to form decomposition products. Calcium cyanamide starts to de 3 compose at temperatures as low as about 250 C.

it may be used at the temperatures usually used for producing nitrided coatings. It may also be used at temperatures below.460 C. as well as at temperatures above 580 C. While it is desirable to use a substance which starts to decompose at temperatures somewhat below the predetermined treating temperatures, it'is sufiicient if one of the decomposition products is produced when the treating temperature is reached. The accelerated action may perhaps be due to double decomposition of the substance, but the exact nature of the reactions is unknown to me.

Solid substances such as urea, alkali cyanides and alkaline earth cyanamides are convenient to use. For example, urea, potassium cyanide and calcium cyanamide give excellent results at treating temperatures between 460 C. and 580 C. A quantity of one or more than one of them may be placed in the container as a solid mixed with the inert material around the article to be treated. In this way they provide an established source of decomposition product next to the surface of the article where the nitrogen-containing decomposition product is to be utilized.

As a specific example of my invention,

1. A steel alloy article containing 95.5% iron, 1% aluminum and small amounts of phosphorus, sulphur, manganese, silicon, nickel, chromium, molybdenum and carbon was covered with powdered calcium cyanamide mixed with an equal volume of asbestos and heated for four hours to about 460 C. in a closed container through which dry ammonia gas was passed. The article was found to have a wear-resistant, nitrogen-containing coating having a hardness corresponding to more than 1000 Brinell.

An alloy article of similar composition was heated under the same conditions but without ammonia. No appreciable hardening by nitri-' fication was produced on the latter article in four hours. Likewise, no appreciable hardness was produced by heating a similar alloy article under the same conditions for four hours in ammonia alone. When a similar alloy article was heated for four hours in the asbestos and ammonia, hard nitrogen-containing coatings were produced but tests indicated that this case was not as thick as those produced when the accelerators were present.

Nitriding with the aid of accelerators appears to produce a greater penetration of the hard nitrogen-containing substance than nitriding without them. No method is known to me of determining the exact thickness of the nitrogen-containing coatings by direct measurements or of determining the exact hardness of the material of the nitrogen-containing case. This is because of the extreme thinness of the coatings and because of the fact that the composition of the coatings gradually approaches the composition of the core material as the distance from the surface increases. Hardness testing machines indicate that harder and firmer cases are formed in a given time with the use of accelerators than without them. It is my present belief that the apparent increase in hardness of the case which is indicated by hardness testing machines, is due to the presence of thicker layers of the nitrogencontaining material but I do not limit my invention to the foregoing theory.

By embedding the article to be nitride-(i in a mixture of the accelerator with other lrniillis inert materials, such as powdered carbon. pow

dered alkaline earth oxides or silica flour, similar results have been obtained. These materials are preferably powdered so as to pass a screen of 200 meshes to the inch but'coarser or finer powders ,may be used. The mixture of accelerator and inert material may be varied within broad limits. About 30 to '10 parts by volume of accelerator are usually used. About equal parts by volume of the accelerator and inert material generally give the best results. Smaller or greater proportions of the accelerator may be used to approach the results obtained by the use only of inert material on the one hand or to approach the results obtained by the use only of ammonia on the other hand. By varying the content of the accelerator, the mixture may be adjusted to a great variety of alloys to be hardened and of temperatures and periods of heating to be used in order to produce the best coatings for special purp OSCS Although I have given only a few specific examples of accelerators and have given one example of the method of using accelerators, it is evident that the use of other accelerators of the kind described which, upon heating, decompose to form a nitrogen-containing gas, as well as modifications of the described method are within the scope of my invention. Therefore, I wish to be limited only by the prior art and by the, invention as defined in the annexed claims.

I claim:

1. The method of producing hard nitr0gen containing cases on ferrous alloy articles which comprises heating the articles to nitriding temperatures in the presence of a nitrogen-containing gas, a powdered inert material and at least one of the following substances which starts to decompose and form a nitrogen-containing gas at temperatures at least as low as said nitriding temperatures: a solid cyanide, a solid amide.

2. The method of producing hard nitrogencontaining cases on ferrous alloy articles which comprises treating the articles by heating to tem peratures of about 460 C. to about 580 C. in the.

3. The method of producing hard nitrogencontaining cases on ferrous alloy articles which comprises treating the articles by heating to temperatures of about 460 C. to about 580 C. in the presence of ammonia and at least one of the following substances: urea, a solid alkali cyanide, a solid alkaline earth cyanamide.

4. A composition for accelerating the action of nitrogen-containing gases in'the production of hard nitrogen-containing cases which comprises finely divided inert material and a solid amide.

5. I} composition for accelerating the action of nitrogen-containing gases in the production of hard nitrogen-containing cases which comprises finely divided inert material and at least one of the following substances: urea, a solid alkaline earth cyanamide.

6. A composition for accelerating the action of nitrogen-containing gases in the production of hard nitrogen-containing cases which comprises about equal parts by volume of a powdered inert material and at least one of the following substances: urea, potassium cyanide, calcium cyanamide.

7. A composition for accelerating the action of nitrogen containing gases in the production of hard nitrogen-containing cases which comprises about equal parts by volume of a powdered inert material and calcium cyanamide.

8. The method of producing hard nitrogencontaining cases on ferrous alloy articles which comprises treating the articles by heating to temperatures of about 460 C. to about 580 C. in the presence of ammonia, a powdered inert material and at least one of the following substances: urea, a solid alkali cyanide, a solid alkaline earth cyanamide.

9. A composition for accelerating the action of ammonia in the production of hard nitrogencontaining cases which comprises finely divided inert materal and at least one of the following substances: urea, a solid alkaline earth cyanamide.

10. A composition for accelerating the action of ammonia in the production of hard nitrogen-conta rting cases which comprises about equal parts by volume of a powdered inert material and at least one of the following substances: urea, potassium cyanide, calcium cyanamide.

JOHN J. EGAN. 

